1. Field of the Invention
The present invention relates to a skew correction device, a sheet handling apparatus, and an image forming system.
2. Description of the Related Art
These days, various fold shapes are employed for sheets, and there is an increasing need to punch their surfaces. In particular, sheets of offset z-fold paper are widely practiced, and punching their surfaces is frequently performed as a matter of course. Because positional accuracy of the punching is essential at this time, many inventions for improving the positional accuracy have been developed.
One aspect of correction for improving the positional accuracy is skew correction. Widely known as the skew correction is a method for correcting skew by causing a sheet to abut on stopped registration rollers, further rotating carriage rollers on the upstream thereof to form flexure, and rotating the registration rollers in the normal direction. However, this method fails to correct skew of z-fold paper stably.
Japanese Patent No. 4016621, for example, discloses a conveying device for providing skew correction without fail even if a sheet is nipped in a nip portion (registration rollers) of a downstream conveying unit, or the sheet gets stuck in a roller element. The conveying device includes a control unit that corrects skew distortion by reversely driving the downstream conveying unit at an operational timing close to an operational timing at which the leading edge of a conveyed body reaches the downstream conveying unit, and cancelling the reverse drive of the downstream conveying unit after a predetermined period of time elapses, and reversely drives the downstream conveying unit on condition that one side of the conveyed body be detected not to abut on the downstream conveying unit nearly evenly.
In the method for performing skew correction by reversely driving the registration rollers under the condition that a sheet be detected not to come into contact with (abut on) the registration rollers evenly as disclosed in Japanese Patent No. 4016621, the number of operations increases compared with the method in which a sheet is caused to abut on stopped registration rollers, thereby reducing the productivity.
In other words, conventionally, skew correction has been performed by reversely rotating the registration rollers on all sheets if skew occurs. In this correction method, however, it takes a long time to rotate the rollers reversely and to rotate the rollers normally thereafter. As a result, the productivity is reduced compared with the skew correction in which a sheet is caused to abut on the stopped registration rollers. In terms of a sheet of unfolded paper, only by causing the sheet to abut on the stopped registration rollers and to form flexure, an advantageous effect of skew correction can be obtained considerably. By contrast, in terms of a sheet of fold paper, such as offset z-fold paper, if the sheet is caused only to abut on the stopped registration rollers, the leading edge of the sheet is likely to be nipped unevenly because of folding defect and the thickness of the sheet. As a result, the advantageous effect of skew correction is less likely to be obtained, and fluctuation in the correction is large.
Therefore, there is a need for technology capable of improving skew correction performance for a sheet on which folding is performed without reducing the productivity of the sheet on which folding is performed.